Chill cladding method and apparatus



Dec. 9, 1969 J. F. CLARKE CHILL CLADDING METHOD AND APPARATUS Filed Dec. 1.9, 1966 YL CAW H R 9 wm EEmw RLW FEO P 09 l \l /N/ I 70 J FIG].

United States Patent() CHILL CLADDING METHOD AND APPARATUS John F. Clarke, Attleboro, Mass, assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Dec. 19, 1966, Ser. No. 603,031

Int. Cl. B05c 5/00; B44d 1/02; C23c 1 U.S. Cl. 117-227 Claims ABSTRACT OF THE DISCLOSURE A length of metal substrate core-forming material is moved through a heating retort containinga protective atmosphere and then up through a melt of metal. A highfrequency A.C. circuit has one connection with the core outside of the melt in the retort and another connection with the melt and hence with the substrate where it enters the melt at a meniscus. As 'a result, a comparatively high surface temperature is effected by use of a comparatively small amount of heat energy. The high temperature of the meniscus surface and of the core Where it engages the meniscus favors efiicient wetting of the core by the melt. The high skin temperature of the substrate in the retort also efficiently cleans it preparatory to said wetting. Upon conduction of the comparatively small skin heat to the center of the substrate while moving through the melt above the meniscus, substantial amounts of metal in a single pass are strongly frozen thereon.

This invention relates to chill cladding and with regard to certain more specific features to continuous chill cladding of a coating on a substrate of core material in the form of a rod, wire, tube or the like. The invention is an improvement upon chill cladding means set forth in the copending U.S. patent application of myself and Clyde M. Adams, Jr., Ser. No. 602,717, filed Dec. 19, 1966.

Among the several objects of the invention may be noted the provision of rapid and low-cost means for chill cladding a coat of material (usually metal) on a metal core or substrate in a manner such that increased thickness of the coat can be clad onthe core than heretofore obtained; the provision of means by which a better bond can be obtained between the cladding and core materials as well as superior surface characteristics of the clad coat; the provision of a stronger finished clad product; and in particular the provision of improved means for cleaning and wetting the core for improving the bond between the core and the coat. Other objects and features will be in part apparent and in part pointed out hereinafter.

In the accompanying drawings which illustrate one various possible embodiments of the invention,

FIG. 1 is a diagrammatic view of apparatus for carrying out the invention; and

FIG. 2 is an enlargement of portions of FIG. 1.

Corresponding reference characters indicate corresponding parts throughout the two views of the drawings.

To avoid circumlocution, in the following the term substrate shall include core material in the form of wires, rods, tubes, ribbons and the like of various cross sections. A

It is known to coat a substrate in the form of a metal core with a metallic cladding of another'metal. For example, core material (for example, 0.125 inch of steel Wire) has been cleaned by immersion in a conventional chemical cleaning solution, then preheating to a predetermined temperature (for example, 900 F.) in a neutral or reducing atmosphere and then immersion in molten meltal at a predetermined temperature (for example,

3,483,030 Patented Dec. 9, 1969 time such that a predetermined amount of molten metal was solidified or frozen onto the solid core material. The cladding thickness was controlled by the preheated temperature of the core, the cladding bath temperature and the residence time of the core in the bath. At the beginning of residence the rate of solidification or freezing of the coating on the core was considerable because of the large temperature gradient between them. However, the temperature gradient rapidly decreased as the bath heated the partially coated wire. There resulted a tendency for the rate of freezing to decrease and sometimes for the bath to remelt the initially solidified coat. The result was a severe limitation upon the amount of cladding that could be applied to the core material.

Other disadvantages of the former processes were the deleterious effects of improper cleaning of the core material and the fact that the hot coating initially came into contact with a relatively cold surface. These conditions resulted in inadequate initial wetting of the solid core material by the molten metal with consequent uneven and unpredictable heat flow. The result was considerable variation in bond strength and thickness of the as-case cladding, both of which undesirably affected the quality of the end product. The present invention overcomes these difficulties.

- In high-frequency electrical heating, the current which produces the heat in the body being heated is concentrated in the outer skin portion of the body. The thickness of the skin in which the current is concentrated is represented by the equation:

p=electrical resistivity (ohm-cm.) of the material,

p.=relative magnetic permeability (ohm-sec./cm.),

f=frequency of the electrical power supply (c.p.s.),

S=skin depth (in which approximately of the current flows) in cm.

In the case of a carbon steel core where p is approx:- mately 10 ohm-crn., and p. is approximately 1,000 ohmsec./cm., if a moderately high frequency such as 500 kilocycles is used, the current will be concentrated within an outer skin depth of approximately 10 microns assuming a inch wire diameter.

Referring now more particularly to the drawing, numeral 1 indicates an upwardly moving continuous supply of high-tensile-strength A; inch diameter steel wire which is to be clad'with a coating of aluminum. An appropriate upward speed is feet per minute. Conyentional'means is employed for moving the Wire and is represented by the dart 2. The aluminum is carried as a melt 3 in a crucible 5 through which the wire moves upward. The wire enters with a running slip fit through a lower opening 4. Movement of the wire through the aluminum forms a meniscus at 6. The fit, while liquid-tight is not gas tight, thus allowing the meniscus to be formed. However, downward leakage of the melt is inhibited. Attached to the crucible 5 under opening 4 is a preheating retort 7 through which the Wire 1 passes before it enters the crucible. Another lower opening 10 admits the wire with a running slip fit. The retort 7 has a gas inlet 8 and a gas outlet 12 for circulating a suitable reducing atmosphere such as hydrogen, cracked ammonia gas or the like, as indicated by the stippling. Some of this gas can reachthe meniscus through the opening 4 along .with the substrate 1. In some cases an inert atmosphere such as helium may be employed. All of such atmospheres are protective, Whether reducing or not.

At numeral 9 is shown a source of A .C., high-frequency electrical power (500 kilocycles, for example). The highfrequency current is applied to the wire 1 over insulated lines 11 and 13. Line 11 extends to a terminal brush or slip contact 15 in the retort 7 which contacts with the moving wire 1. Line 13 is connected with the terminal 15 which is engaged by the melt 3 in the crucible 5. Thus current connections are with the wire 1 at brush 15 and at point 19 where the wire first becomes contacted by the meniscus 6 of the melt 3. The current flows through the melt 3 and to the power source 9 via terminal 15 and connection 13. This arrangement constitutes an improvement over that shown in said copending application wherein current is applied to and taken off from the wire 1 by means of two brushes in the retort 7, one of which must be positioned quite close to the inlet 4. The present invention has the advantage that only one sliding brush such as 15 is used and a second brush is eliminated. In its stead there is the connection between wire 1 and the melt 3 at the upper cusp formed by the meniscus 6 at 19. Thus the wire 1 is heated by the high-frequency current throughout all of the distance 1 between brush 15 and the contact point of wire 1 with the melt at point 19. As a result, no cooling of the wire occurs before the wire enters the melt 3. Moreover, the wire remains very hot right up to point 19. The high-frequency current flow will be in a thin skin of the wire and in the skin of the meniscus, as illustrated by the dotted lines C in FIG. 2. With a power input of 8 kilowatts, for example, a desired skin temperature may be reached before the substrate reaches the melt such as 1250 F.-l400 F. or more. This range is considerably higher than the melting point of the aluminum in melt 3, which is about 1220 F. and results in superior cleaning of contamination from the surface of wire 1. Moreover, any small loose contamination that may remain at 19 tends to be Washed away by turbulence at that point. Such washed-away material is harmless in the melt 3.

The frequency and power of the source 9, along with the speed of a wire 1 of a certain diameter and material, are selected so that first, the desired skin temperature (1250 F.1400 F. or more) of the wire as it enters the melt 3 will be high enough so as first to effect good cleaning of the Wire surface by evaporating contaminants and reducing or preventing formation of oxides by the action of the atmosphere in retort 7; second, the skin temperature of the wire as it enters the bottom of the melt 3 will be considerably higher than the melting point (say 1220 F.) of the aluminum melt; and, third, during the residence time of the wire in the melt the heat from its skin shall have spread throughout the cross section and bulk of the substrate to reduce its temperature (900 F., for example) below the melting point of the melt. As a result of the above, the initial contact of the melt with the wire is on a very clean surface under excellent wetting conditions before any solidification or freezing of the melt on the wire. Thereafter, as the wire ascends in the melt, the heat of the skin of the wire will spread throughout the bulk mass of the wire, whereby the wire temperature will become reduced below that of the aluminum melt and below the melting point of aluminum by the time the wire reaches its point of exit from the surface of the melt. This favors efiicient freezing onto the wire of a thick coat, without remelt. Thus an important feature of the invention is first to have the wire 1 enter the melt in as hot and clean condition of its skin as possible above that of the melting point of the melt and, second, to have its bulk temperature become reduced only after and not before the wire enters the melt. It is in this regard that the present invention differs from that of said application. Thus even a better bond is obtained between the wire and the melt than set forth in said application. And as set forth in said application, remelting is minimized by having the reduction in the wire temperature for freezing action delayed until the wire is immersed in the melt. As a result, thicker coating can be obtained than heretofore. Another advantage is the elimination of the contact materials problem where the upper contact of a two contact system is rubbing on hot metal thus seriously limiting its life.

By only skin-heating the wire core at the high temperature in the retort 7, high-temperature annealing of the wire is greatly minimized, thus substantially maintaining its strength as originally manufactured.

As set forth in said application, the relationship for substantially maintaining the heat in the skin portion of the wire until immersion in the melt is:

where: a thermal diffusivity of the material (in. per sec.) r=radius of the wire in inches t=elapsed time in seconds If the wire is traveling at a velociyt v, the distance I between 15 and 19 (in order to maintain the preheat in the skin) is given by:

where:

l=the distance between 15 and 19 in inches r=the radius of the wire in inches v=wire velocity in inches per second x=therma1 diffusivity of the wire material in in. per

second Employing the above formulations, for steel wire 0.250 inch in diameter, traveling at feet per minute, the distance between 15 and 19 theoretically will be approximately 0.3 inch to maintain preheating of the skin. In practice, however, this distance may be increased somewhat.

It will be seen fromthe above that my improvement over the cladding means disclosed in said application comprises making one of the electrical contacts with the Wire 1 through the one sliding contact 15 and the second electrical contact therewith by means of the molten metal itself in the casting zone beginning at point 19.

It will be understood that while the core material has been described above as being in the form of a wire, rod, tube ribbon or the like, and that its designation as a substrate is intended in the following claim to gen generic to these and equivalent forms.

While it is preferred, as above described, that the skin temperature of the substrate 1 exceed the melting point of the melt 3 in order to provide improved wetting, some advantages other than wetting still accrue if this temperature is somewhat less than the melting point of the melt. Thus the temperature range of 1250 F.-1400 F. mentioned above for the skin tempertaure might be in the range of 1200 F.-1220 F. It will be understood that the skin temperature may be higher than 1500 F., but no particular advantage is achieved by the additional heating.

The temperature range of the aluminum melt extends from a point sufficiently above the melting point of aluminum to prevent inadvertent chilling to 1500 F. or above depending upon the cladding thickness desired.

In View of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

What is claimed is:

1. The method of chill clading a melt of a metal coating material on a length of electrically conductive substrate, comprising moving the length of substrate into the bottom and upwardly through the melt to form a meniscus at said bottom, passing a current of A.C. high-frequency electrical energy from an electrical source between a point on the substrate outside of the melt to the point where the substrate first contacts the melt at said meniscus as the substrate moves into the melt, said electrical energy substantially heating and raising the skin temperature of the substrate and of the meniscus to a degree higher than that of the melt, limiting the heat energy supplied to the skin from said source so that upon spreading through a 5 bulk of the material in the substrate in the melt, the substrate acquires a temperature below the melting point of the melt to cause freezing of the melt on the substrate.

2. The method according to claim 1, wherein the substrate is a continuous length of steel and the coating material is aluminum.

3. The method according to claim 2, wherein the substrate moves at a speed of approximately 100 feet per minute, wherein the skin temperature of the wire in approach to the melt becomes on the order of 1250 F.1400 F., and wherein the temperature of the substrate in the melt becomes on the order of about 900 F.

4. Chill cladding apparatus comprising a container for a melt of conductive coating material, said container having a lower inlet for receiving into the melt a length of a conductive metal substrate, means for moving said substrate via said inlet upwardly into, through and out of the melt, said melt forming a meniscus around the substrate at the inside of the inlet, an AC. high-frequency electrical energy source, an electrical connection between said source and the melt and an electrical connection between the source and a slip contact engaging the moving substrate outside of the inlet, the energy and frequency of said source substantially heating only a substantially thin skin portion of the meniscus and a substantially thin skin portion of a length of the substrate moving between the slip contact and the point of engagement of the substrate with the melt at the meniscus to a temperature above that of the remainder of the substrate, the 'bulk of the substrate in the melt absorbing heat from said skin portion to lower the temperature of the substrate below the melting point of the melt.

5. Chill cladding apparatus comprising a crucible for a melt of metal, said crucible having a lower inlet for receiving a moving length of a metal substrate with a sliding fit to seal against leakage of the melt but not against gas, a retort under said crucible and connected with said inlet, a protective gas in the retort, means for moving said substrate upwardly through said inlet and the melt, said melt forming a meniscus around the substrate within said inlet and to which said gas is accessible, a slip contact engaging the substrate outside of the inlet, an AC. highfrequency electrical power source, an electrical connection between the source and the melt, an electrical connection between the source and said slip contact, said power source substantially heating only a substantially thin skin portion of the meniscus and of the substrate over a length of the latter between said slip contact and the meniscus, the energy of said source raising the temperature of said thin skin portions to a degree higher than the melting point of the melt to improve wetting of the substrate by the melt, the bulk temperature of a length of the substrate in the melt attaining a temperature lower than the melting point of the melt.

References Cited UNITED STATES PATENTS 2,320,801 6/1943 Simons 118--620 2,926,103 2/1960 Brich 117-1l4 3,227,577 1/1966 Baessler et al 117-114 3,410,715 11/1968 Hough 117---114 ALFRED L. LEAVITT, Primary Examiner W. E. BALL, Assistant Examiner US. Cl. X.R. 

